There are a variety of apparatuses for measuring, the concentration of certain gaseous components of a gas mixture. Simple apparatuses, referred to as gas detector tubes, colorimetric tubes, or gas indication tubes (“gas detector tubes”), typically comprise a transparent tube and a chemical reagent within the transparent tube that is capable of reacting with a target chemical compounds resulting in a color change of the reagent. In a typical colorimetric gas detector tube, a known volume of air is passed through the tube with a pump or other device. The chemical reagent indicates the presence of target compounds by changing color from the inlet end of the tube. The resultant length-of-stain (the length of the color changed section of the chemical reagent) depend on the concentration of the target compounds in the gas which has passed through the tube in the known volume of gas. Colorimetric gas detector tubes are used throughout industry as a low-cost and easy-to-use tool for detecting the presence of target compounds in a sampled volume of gas. The tube is typically, made of glass or another transparent material so that the length-of-stain may be measured.
For example, conventional gas detector tubes comprise a glass tube filled with a chemical reagent that reacts to a specific target chemical compounds. The chemical reagent is sealed within the glass tube and retained in a defined position by gas permeable plugs in both ends of the glass tube. In some cases, the chemical reagent may be liquid impregnated into a porous chemically neutral solid substrate. Prior to use, the chemical reagent is protected from exposure to contaminants and chemical compounds by sealing the ends of the gas detector tubes into tips until use, thereby extending the shelf life of the gas detector tube prior to use. To use the gas detector tube, the tube on both ends of the detector tube are broken off to open a gas flow path through the tube and across the reagent. The air or other gas to be sampled may then be drawn through the tube and into contact the reagent with a fixed volume of a sample drawn through a volumetric sampling pump, for example. The reagent is capable of rapidly reacting with the target compounds as the sample is drawn through the tube. The amount of reaction and the degree of change of color of the reagent are related to the concentration of the target chemical compounds in the sampled gas, the amount of reagent in the tube, the flow area of the gas tube, and the volume of gas drawn through across the reagent. Since the sampled gas is drawn in one end of the gas detector tube and out the other end, the reagent begins to change color at the inlet end and the color change extends toward the outlet. The total length of the color change is commonly referred to as the “length-of-stain.”
To determine the concentration of a target compound, a known volume of the sample gas may be drawn into the gas detector tube comprising a known quantity of reagent that reacts in a repeatable manner resulting in a color change with target compounds. After sampling the length-of-stain should correlate to the only unknown variable, the concentration of the gas. The length of the color change and the degree of color change of the reagent then corresponds to the concentration of the target compounds. Detector tubes that measure gas concentration by length-of-stain are reliable and simple to use after training.
To ensure more accuracy in measuring the concentrations of target gases, after manufacturing a batch of gas detector tubes, fixed volumes of gas with known concentrations of target compounds are passed through the gas detector tubes to develop a batch specific calibration curve relating the length-of-stain to corresponding gas concentration. The calibration curve is included with the detector tube to allow visual reading of the concentration of a gas in a sampled volume. From their first introduction the detector tubes have their scale printed separately, see, for example, U.S. Pat. No. 2,174,349 to J. B. Littlefield. As the leading edge of color change (closest to the exit end of the tube) of the reagent in the detector tubes is not always well defined, the scale divisions may be marked having a distance greater than length of diffusive front of discoloration. As such, the scales are of poor resolution and, more recently, the scales printed directly on the surface of the tube. For example, Dreager™, Gastec™, Kitagava™, Auer™, MSA™, RAY™, as well as other manufacturers of detector tubes have on their tubes the beginning of the scale first scale division, marked 3 to 5 mm from the end the first input plug. See FIG. 1. Because of possible channeling effects and other flow dynamics, resulting in different lengths visible on each side of the tube, the divisions are printed as rings around the tube and numbers representing concentrations are in close proximity or into broken portions of a ring. The drawback of the known art is that such detector tubes cannot be accurately and repeatedly read by an electronic device because of the concentration lines, concentration amounts, and other marks on the tubes obstructing optical reading of the length-of-stain. For example, the markings may be incorrectly interpreted by the device as a color change of the chemical reagent.
There has been a long felt need for a better more accurate and objective way of reading gas concentration with gas detector tubes. Heim et al, in U.S. Pat. No. 4,123,227 show a length-of-stain tube electronic reader based on detector tube without any printed matter. The detector tube serves as an alarming device and is periodically interrogated over a period of time. Leichnitz is at in U.S. Pat. No. 5,069,879 suggested tube having no scale on the readable part, of the surface and printed means introducing into electronic reader all specific data for the tube including calibration data. There is a significant drawback of the tunes manufactured to be read by electronic reader only; they may not also by read visually due to their lack of demarcations.
The contemporary art of colorimetric reading devices is developed in the direction of devices even more specifically designed for optic-electronic reader. U.S. Pat. No. 5,089,232 to May shows an arrangement of tube-like devices for only electronic reading. U.S. Pat. No. 5,197,538 to Stark et al., U.S. Pat. No. 5,415,838 to Rieger et al. and U.S. Pat. No. 5,464,588 to Bather at al. depict development of specific tube-like devices electronic reading of a zone of discoloration. Such devices, however, are highly specific and cannot be read without specialized electronic means.
Most color changes indicated by colorimetric reactions of the reagent depend to some extend on the ambient conditions such as temperature, relative humidity and barometric pressure.
There exists a need for an apparatus, kit, device, and method for visually or manually reading colorimetric detector tubes that do not include concentration demarcation marks.